The present invention relates to an optical communication system comprising a transmission switching system.
More particularly, the present invention relates to a system and a method of transmission switching for an optical communication system comprising at least a first primary guided optical path for the transmission of at least one optical signal and at least a first secondary guided optical path to which the transmission of the optical signal can be switched in case of degradation of the transmission in the first primary guided optical path.
In the operation of optical communication systems there is a widely felt need to minimize the problems which arise when there is a deterioration of transmission due, for example, to a fault of a device in a guided optical path (e.g. an optical amplifier) and/or of a device in a terminal station (e.g. a transmitter or receiver) and/or to the breaking of an optical cable.
Among the operating systems for optical communication systems, there are known remote monitoring systems for detecting and locating the presence of a fault in the system.
EP 0 408 905 describes an optical fibre telecommunications line comprising active optical fibre amplifiers. Each active optical fibre present in the amplifiers is connected to two laser sources of optical pumping radiation. The first of these two laser sources of optical pumping radiation is put into operation and the second is kept in reserve so that it can be put into operation when the first becomes faulty. The two sources of optical pumping radiation are also connected to a microprocessor circuit capable of commanding them to send alarm signals on the state of the amplifier to the terminal stations of the line and to receive from these stations control signals for switching operation between the two sources of optical pumping radiation.
U.S. Pat. No. 5,475,385 describes a telemetry system for locating a fault in an optical transmission system including optical fibre amplifiers. Each amplifier has a corresponding source of pumping radiation comprising a loop system, for automatic gain control, around the fibre of the optical amplifier. The spontaneous emission of each amplifier is detected and used to control the corresponding source of pumping radiation and to maintain the output of the amplifier at a predetermined level. The transmitter of the system injects a monitoring tone whose presence or absence is detected by each amplifier. If there is a fault in the transmission line, the down-line amplifiers, not receiving the said monitoring tone, inject a corresponding alarm tone. In reception, suitable devices detect the presence or absence of the monitoring tone and, in its absence, count the number of alarm tones, if they are present, thus determining the position of the fault. The absence of the monitoring tone and of the alarm tones indicates that the fault has occurred between the last amplifier and the receiving station.
U.S. Pat. No. 5,483,233 also describes a telemetry system for locating a fault in an optical transmission system including optical fibre amplifiers. Each amplifier comprises an active optical fibre with a loop system for automatic gain control (AGC) around it. In the AGC the spontaneous emission from the amplifier fibre is detected and used to control the source of pumping radiation in such a way as to maintain the output of the amplifier at a predetermined level. A monitoring tone is injected by the transmission unit of the system. Each amplifier detects the presence of the monitoring tone or of an alarm tone originating from the preceding amplifier. If one of the tones is detected, the signal is amplified and transmitted to the next amplifier. If no tone is detected, an alarm tone corresponding to the amplifier tone is injected into the corresponding pumping source. At the receiving unit of the system, the presence or absence of the monitoring tone, and in its absence the presence or absence of an alarm tone, is detected. The position of a fault in the system is determined from this information.
EP 0506163 describes an optical fibre telecommunications line, comprising two terminal stations, each having a transmitter of optical signals and a receiver connected operationally by an automatic protective device capable of switching off the transmitter in the absence of a received optical signal, and corresponding optical fibre lines connecting the transmitter of one station to the receiver of the other station and including at least one optical amplifier. At least one of the optical amplifiers has a protective device comprising means of detecting the presence of an optical signal at the output of the amplifier. The interruption of light emission in the down-line direction causes the interruption of emission in the whole line by the intervention of the automatic protective device of the terminal stations.
EP 0507367 describes an optical fibre telecommunications system comprising at least one amplifier with an active fibre having a fluorescent element and a laser pump for supplying light energy to the active fibre. Detection means for detecting the presence of light energy are connected to the input of the active fibre. In the absence of light energy at the input of the active fibre, control means are put into operation by the said detecting means to interrupt the pump source in order to interrupt substantially any emission of light energy from the optical amplifier.
In addition to remote monitoring systems for detecting and locating the presence of a fault in the system, there are known remote operating systems for switching, in case of a fault, the transmission of an optical signal, at a predetermined frequency, from a first guided optical path to a second guided optical path.
U.S. Pat. No. 5,091,796 describes a communication system comprising a plurality of stations, a first active line (guided optical path) which connects adjacent stations of a plurality of stations, a second active line which by-passes predetermined stations without xe2x80x9cdroppingxe2x80x9d into them, and a third line for protection, in common between the first active line and the second active line. A monitoring signal travels continuously along the protection line. Each station includes an internal circuit for receiving an input transmission signal from the first active line and for sending an output transmission signal along this line. Each station also includes a switching section for the passage of the protection line without allowing it to drop into each station in normal conditions, and for connecting, in alarm conditions, the protection line to the internal circuit so that the input transmission signal is received by the protection line up-line instead of by the first active line, so that the protection line up-line from the station is used, instead of the first active line in which, for example, a fault has occurred. Additionally, the monitoring signal, which is generated by the output transmission signal, is sent, down-line, through the protection line.
In the said known systems, a degradation of transmission is dealt with at the transmitting and receiving stations and also at intermediate stations or optical amplifiers along a guided optical path. These therefore require a single operating system which is perfectly compatible with the guided optical path and with the transmitting and receiving terminal stations which must, therefore, communicate with each other.
The inventors have perceived that, in practice, the manufacturers of terminal stations are frequently different from the manufacturers of guided optical paths, comprising optical cables and optical amplifiers or intermediate stations. The said conventional systems therefore require, at the time of assembly of an optical communication system, a laborious process of adaptation for making the terminal stations compatible (communicating) with the guided optical path.
The inventors have also perceived that, in the said conventional operating systems, in the case of a multiple-wavelength transmission in a single guided optical path, when a fault occurs along the guided optical path and it is therefore necessary to switch the transmission of all the transmitted wavelengths to another guided optical path, the switching should be controlled by as many terminals as there are wavelengths transmitted. Each terminal should therefore check for the presence of any faults in the guided optical transmission path and, if necessary, switch the transmission of the corresponding wavelength. This would require a transmission switching system which is complex and consequently expensive and unreliable.
In the particular case of loop networks provided with a plurality of nodes, it has been proposed to provide an additional protection ring (closed optical path) and to propagate an optical signal simultaneously in both loops in two different directions of propagation.
EP 0769859 describes a fail-safe loop optical communication network comprising a first optical communication line, forming a closed optical path; at least two nodes for injection and extraction of optical signals connected optically along the line; a second optical communication line, forming a closed optical path and connected optically to the nodes for injection and extraction of optical signals. In the network there are defined a first and a second direction, opposite to each other, of travel of the optical signals with respect to the position of the nodes for injection and extraction of optical signals. At least one of the said nodes also comprises selection means, controlled by the optical signals, for the selective extraction of the optical signals from one of the communication lines. At least one of the nodes for injection and extraction of optical signals also comprises means for the simultaneous injection of at least one optical signal in the first direction along the first communication line and in the second direction along the second communication line.
The object of the present invention is to overcome the aforesaid disadvantages of the conventional operating systems. This is achieved by means of an optical communication system comprising a transmission switching system which is universal and independent of the transmitting and receiving terminal stations.
In a first aspect, the present invention provides therefore an optical communication system comprising:
a first transmitter;
a first receiver;
a first primary guided optical path having a protected portion;
a first secondary guided optical path;
at least an optical amplifier;
a modulation device, up-line from the said protected portion of the said first primary guided optical path, for transmitting a first overmodulation frequency along the said protected portion;
a detection device, down-line from the said protected portion of the said first primary guided optical path, for detecting the presence or absence of the said first overmodulation frequency; and
an operating device, down-line from the said protected portion of the said first primary guided optical path, connected operationally to the said first secondary guided optical path;
the said first transmitter sending at least one optical signal at least along the said first primary guided optical path and the said first receiver receiving the said at least one optical signal, in which, when the said detection device detects the absence of the said first overmodulation frequency in the said protected portion of the said first primary guided optical path, the said operating device enables the said optical signal to be propagated along the said first secondary guided optical path up to the said first receiver.
In the present description and in the attached claims, the expression xe2x80x9cguided optical pathxe2x80x9d is used to mean a path which physically connects separate points of a system and/or of an optical communication network, and which is capable of transferring an optical signal in a guided way from one to another of the said points. This path may comprise optical waveguides such as optical fibres and optical amplifiers, of the doped fibre type for example.
The expression xe2x80x9cprotected portionxe2x80x9d is used to mean any section of a guided optical path which is to be protected against any fault in the said guided optical path.
Preferably said protected portion is comprised between a WDM multiplexer and a WDM demultiplexer.
Typically, said WDM multiplexer is located up-line from said modulation device.
Typically, said WDM demultiplexer is located down-line from said detection device.
In one embodiment, the said at least one optical signal is also sent along the said first secondary guided optical path and, when the said detection device detects the presence of the said first overmodulation frequency in the said protected portion of the said first primary guided optical path, the propagation of the said optical signal along the said first secondary guided optical path is blocked before the said first receiver.
In another embodiment, the said optical communication system also comprises a second primary guided optical path having a protected portion, a second transmitter for sending at least one second optical signal at least along the said second primary guided optical path, and a second receiver for receiving the said at least one second optical signal.
Preferably, the said second primary guided optical path is operationally connected to the said first primary guided optical path.
Advantageously, the said at least one second optical signal has a propagation direction opposite that of the said at least one optical signal.
Preferably, the optical communication system also comprises a second secondary guided optical path, connected operationally to the said second primary guided optical path.
More preferably, the said second secondary guided optical path is also operationally connected to the said first secondary guided optical path.
In a first embodiment, the said first primary guided optical path and the said first secondary guided optical path are connected by a first beam splitter up-line from the said protected portion of the said first primary guided optical path.
In a second embodiment, the said first primary guided optical path and the said first secondary guided optical path are connected by a first coupler down-line from the said protected portion of the said first primary guided optical path.
In a third embodiment, the said second primary guided optical path and the said second secondary guided optical path are connected by a second beam splitter up-line from the said protected portion of the said second primary guided optical path.
In a fourth embodiment, the said second primary guided optical path and the said second secondary guided optical path are connected by a second coupler down-line from the said protected portion of the said second primary guided optical path.
In a fifth embodiment, the said first primary guided optical path and the said first secondary guided optical path are associated with a first switch up-line from the said protected portion of the said first primary guided optical path.
In a sixth embodiment, the said first primary guided optical path and the said first secondary guided optical path are associated with a second switch down-line from the said protected portion of the said first primary guided optical path.
In a seventh embodiment, the said second primary guided optical path and the said second secondary guided optical path are associated with a third switch up-line from the said protected portion of the said second primary guided optical path.
In an eighth embodiment, the said second primary guided optical path and the said second secondary guided optical path are associated with a fourth switch down-line from the said protected portion of the said second primary guided optical path.
Advantageously, at least one of the said first and second primary guided optical paths comprises, in transmission, an optical transmission amplifier.
Preferably, at least one of the said first and second secondary guided optical paths comprises, in transmission, an optical transmission amplifier.
More preferably, at least one of the said first and second primary guided optical paths comprises, in reception, an optical receiving amplifier.
Even more preferably, at least one of the said first and second secondary guided optical paths comprises, in reception, an optical receiving amplifier.
Typically, the said modulation device comprises a modulator associated with each of the said first and second primary guided optical paths to generate the said first overmodulation frequency along the said first and the said second primary guided optical path. Advantageously, the said modulator is associated with the optical transmission amplifier of the said first and second primary guided optical paths.
Preferably, the said detection device is associated with each of the said first and second primary guided optical paths to detect the presence or absence of the said first overmodulation frequency along the said first and second primary guided optical paths. More preferably, the said detection device is associated with the optical receiving amplifier of the said first and second primary guided optical paths.
Advantageously, the said operating device comprises an electronic circuit associated with each of the said first and second primary guided optical paths to switch the transmission of at least one of the said first and second optical signals according to the presence or absence of the said first overmodulation frequency in the said first and second primary guided optical paths. Preferably, the said operating device is associated with the optical receiving amplifier of the said first and second primary guided optical paths.
In one embodiment, the said optical communication system comprises a first plurality of transmitters for sending in the said first primary guided optical path a plurality of optical signals, each having a different wavelength from the other optical signals.
In a variant, the said optical communication system also comprises a second plurality of transmitters for sending in the said second primary guided optical path a plurality of optical signals, each having a different wavelength from the other optical signals.
In another embodiment, the said modulation device also comprises a modulator associated with each of the said first and second secondary guided optical paths to generate a second overmodulation frequency along the said first and second secondary guided optical paths.
Preferably, the said detection device is also associated with each of the said first and second secondary guided optical paths to detect the presence or absence of the said second overmodulation frequency along the said first and second secondary guided optical paths.
A second aspect of the present invention is a method for switching the transmission of an optical signal from a primary guided optical path to a secondary guided optical path, comprising the phases of:
a) sending at least a first part of the power of the said optical signal in the said primary guided optical path;
b) sending an overmodulation frequency in the said primary guided optical path;
c) detecting the presence or absence of the said overmodulation frequency in the said primary guided optical path;
d) allowing the propagation of the said optical signal along the said secondary guided optical path when the absence of the said overmodulation frequency is detected.
In one embodiment, the phase a) also comprises the sending of a second part of the power of the said optical signal in the said secondary guided optical path, and the said method also comprises a phase e) in which the propagation of the said optical signal in the said secondary guided optical path is blocked when the presence of the said overmodulation frequency is detected in the said primary guided optical path.
In one variant, at least the said secondary guided optical path comprises an optical receiving amplifier. Preferably the phase e) is executed by disabling the said optical receiving amplifier. Advantageously, the phase d) is executed by activating the said optical receiving amplifier.
In another variant, the said primary guided optical path and the said secondary guided optical path are associated with a switch. Preferably, the phase e) is executed by closing the said switch on the said primary guided optical path. Advantageously, the phase d) is executed by closing the said switch on the said secondary guided optical path.